An additive manufacturing technology for the fabrication and characterization of nuclear reactor fuel

ABSTRACT

Nuclear fuel structures and methods for fabricating are disclosed herein. The nuclear fuel structure includes a plurality of fibers arranged in the structure and a multilayer fuel region within at least one fiber of the plurality of fibers. The multilayer fuel region includes an inner layer region made of a nuclear fuel material, and an outer layer region encasing the nuclear fuel material. A plurality of discrete multilayer fuel regions may be formed over a core region along the at least one fiber, the plurality of discrete multilayer fuel regions having a respective inner layer region of nuclear fuel material and a respective outer layer region encasing the nuclear fuel material. The plurality of fibers may be wrapped around an inner rod or tube structure or inside an outer tube structure of the nuclear fuel structure, providing both structural support and the nuclear fuel material of the nuclear fuel structure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. Patent Application Ser. No.62/015,603, filed Jun. 23, 2014, entitled “An Additive ManufacturingTechnology for the Fabrication and Characterization of Nuclear ReactorFuel”, and is related to U.S. Patent Application Ser. No. 62/099,734,filed Jan. 5, 2015, entitled “An Additive Manufacturing Technology forthe Fabrication and Characterization of Nuclear Reactor Fuel”, and isrelated to U.S. Patent Application Ser. No. 62/133,596, filed Mar. 16,2015, entitled “An Additive Manufacturing Technology for the Fabricationand Characterization of Nuclear Reactor Fuel”, and is related to U.S.Patent Application Ser. No. 62/153,715, filed Apr. 28, 2015, entitled“An Additive Manufacturing Technology for the Fabrication andCharacterization of Nuclear Reactor Fuel”. Each of these applications ishereby incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

Nuclear energy continues to be an important source of energy for theUnited States and many countries around the world, as nuclear fuel canprovide greater amounts of energy over long time periods without many ofthe problems associated with fossil fuel use, such as greenhouse gasemissions. The inherent risks in using and storing nuclear fuel sources,the need for ensuring safe operation of nuclear reactors, and the risksof nuclear fuel being misused to create weapons continue to driveinnovation in developing safe and secure nuclear fuel technologies.

BRIEF SUMMARY

Various shortcomings of the prior art are overcome, and additionaladvantages are provided through the provision, in one aspect, of anuclear fuel structure which includes a plurality of fibers arranged inthe nuclear fuel structure and a multilayer fuel region within at leastone fiber of the plurality of fibers. The multilayer fuel regionincludes an inner layer region with a nuclear fuel material, an outerlayer region encasing the nuclear fuel material.

In another aspect, also provided is a method of facilitating fabricatinga nuclear fuel structure, where the facilitating fabricating includesproviding a plurality of fibers arranged in the nuclear fuel structureand forming a multilayer fuel region within at least one fiber of theplurality of fibers. The multilayer fuel region formed includes an innerlayer region having a nuclear fuel material, and an outer layer regionencasing the nuclear fuel material.

Additional features and advantages are realized through the techniquesof the present invention. Other embodiments and aspects of the inventionare described in detail herein and are considered a part of the claimedinvention.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

One or more aspects of the present invention are particularly pointedout and distinctly claimed as examples in the claims at the conclusionof the specification. The foregoing and other objects, features, andadvantages of the invention are apparent from the following detaileddescription taken in conjunction with the accompanying drawings inwhich:

FIG. 1 depicts an exemplary layout of a nuclear reactor, illustratinggenerally the use of nuclear fuel structures in nuclear reactoroperation;

FIGS. 2A-2B depict an example structure including multiple nuclear fuelrod structures and control rod structures, illustrating one possiblestructure for encasing and storing nuclear fuel structures;

FIGS. 3A-3B depict one embodiment of a portion of a nuclear fuelstructure or cladding structure, in accordance with one or more aspectsof the present invention;

FIG. 4A depicts one embodiment of a fiber including a multilayer fuelregion, in accordance with one or more aspects of the present invention;

FIG. 4B depicts another embodiment of a fiber including a multilayerfuel region, in accordance with one or more aspects of the presentinvention;

FIG. 4C depicts a cross-section view of a fiber including a multilayerfuel region, in accordance with one or more aspects of the presentinvention;

FIG. 4D illustrates one embodiment of a plurality of fibers including aplurality of discrete multilayer fuel regions, or a plurality ofscaffolding fibers thereof, in accordance with one or more aspects ofthe present invention.

FIG. 5A depicts one embodiment of a portion of a nuclear fuel structurethat includes a plurality of fibers that have multiple discretemultilayer fuel regions, and/or nuclear poison regions, in accordancewith one or more aspects of the present invention;

FIGS. 5B-5E depict cross-sectional views of embodiments of nuclear fuelassemblies using the nuclear fuel structure of FIG. 5A, in accordancewith one or more aspects of the present invention;

FIGS. 6A-6E depict a process for facilitating fabricating a fiber with amultilayer fuel region, in accordance with one or more aspects of thepresent invention;

FIGS. 7A-7E depict another process for facilitating fabricating a fiberwith a multilayer fuel region, in accordance with one or more aspects ofthe present invention;

FIG. 8 depicts one embodiment of an apparatus for facilitatingfabricating a plurality of fibers having multiple discrete fuel regions,in accordance with one or more aspects of the present invention;

FIG. 9 depicts one embodiment of a process for forming a plurality offibers, in accordance with one or more aspects of the present invention;

FIG. 10 depicts one embodiment of a plurality of scaffolding fibers thatmay be formed by a digital spinneret technology, in accordance with oneor more aspects of the present invention; and

FIG. 11 depicts one embodiment of a nanoporous carbon layer, inaccordance with one or more aspects of the present invention.

DETAILED DESCRIPTION

Aspects of the present invention and certain features, advantages, anddetails thereof, are explained more fully below with reference to thenon-limiting examples illustrated in the accompanying drawings.Descriptions of well-known materials, fabrication tools, processingtechniques, etc., are omitted so as not to unnecessarily obscure theinvention in detail. It should be understood, however, that the detaileddescription and the specific examples, while indicating aspects of theinvention, are given by way of illustration only, and are not by way oflimitation. Various substitutions, modifications, additions, and/orarrangements, within the spirit and/or scope of the underlying inventiveconcepts will be apparent to those skilled in the art from thisdisclosure.

Reference is made below to the drawings, which are not drawn to scalefor ease of understanding, wherein the same reference numbers usedthroughout different figures designate the same or similar components.

Nuclear energy production continues to be important in many countries tomeet current and predicted future energy demands. Nuclear fuelmaterials, such as uranium and uranium-based compounds, have a muchgreater energy density than other energy sources such as fossil fuels,and may have energy densities over a million times greater than, forexample, gasoline or coal fuels. Safe handling and storing of nuclearfuel materials within nuclear reactors, as well as prevention of nuclearreactor accidents and meltdowns, continue to be important issues innuclear energy production, as several well-known nuclear reactoraccidents illustrate, such as the Three Mile Island incident, theChernobyl disaster, and the recent Fukushima Daiichi meltdown.

FIG. 1 is a simplified schematic of an example nuclear reactor 100, asmay be part of a nuclear power plant. The nuclear reactor 100 depictedin FIG. 1 includes a reactor vessel 105 encased inside a reactor shield110, which may be made of concrete or other material capable ofwithstanding high temperatures, so that material within the reactorshield 110 may be contained in the event of an accident. Reactor vessel105 includes a core 120 in which nuclear fuel rods 130 and control rods140 are disposed. Reactor vessel 105 also holds a coolant material 150,such as water or heavy water, which may be drawn into reactor 105through a coolant inlet 155. Fuel rods 130 include a nuclear fuelmaterial, frequently uranium dioxide fuel capsules, encased in a metalalloy fuel rod casing or cladding, such as a zirconium alloy metalcasing. (Many nuclear fuel rods make use of zirconium alloy claddingmaterials produced under the trademark name Zircaloy.) The nuclear fuelmaterial is bombarded with neutrons that can initiate a fission reactionin the nuclear fuel; the reaction splits the nucleus of the nuclear fuelmaterial, releasing heat energy and additional neutrons thatsubsequently continue the fission reaction. The heat energy heats thecoolant 160, which may then be pumped out of reactor vessel 105 viacoolant outlet 165; the heated coolant 160 may be used, for example, togenerate steam to drive turbines that in turn generate electrical energy(not depicted in FIG. 1 for simplicity). Core 120 may be made of one ormore moderator materials, such as graphite, capable of slowing neutronsdown to speeds at which the neutrons are more likely to react with thenuclei of the nuclear fuel material. Coolant material 150 may also actas a moderator material to slow down high-speed neutrons bombarding fuelrods 130. Control rods 140 may be used to variably control a fissionrate of the nuclear fuel material in fuel rods 130. Control rods 140 maybe made of one or more materials capable of absorbing neutrons withoutundergoing a fission reaction, such as boron, silver, cadmium, and/orindium. As control rods 140 are pulled out partially or fully from thereactor vessel 105, more neutrons may successfully bombard and reactwith nuclear fuel material in fuel rods 130, increasing energy output;conversely, by inserting the rods further or fully into reactor vessel105, more neutrons may be absorbed and the nuclear fission reactionsslowed to reduce energy production. In some nuclear reactors, fullyinserting control rods 140 may be used to fully stop fission reactionsin fuel rods 130.

FIGS. 2A and 2B illustrate an example of a nuclear fuel assembly 200, asmay be deployed in a nuclear reactor. Generally, nuclear fuel rods 130and control rods 140, as depicted in FIG. 1, are not deployed separatelyin a nuclear reactor, but are more often deployed in a nuclear fuelassembly such as nuclear fuel assembly 200. Nuclear fuel rods 210 may bearrayed together with control rods 220 interspersed among the nuclearfuel rods 210, and both nuclear fuel rods 210 and control rods 220 boundby one or more spacers 230. The entire nuclear fuel assembly 200 may bedeployed within a nuclear reactor vessel, such as reactor vessel 105 ofFIG. 1, so that the nuclear fuel assembly is surrounded by moderatormaterials, such as the core 120, and coolant 150 may flow around fuelrods 210 and control rods 220. The control rods 220 may be coupled withcontrols within or outside the reactor vessel so that the control rods220 may be variably withdrawn or inserted further into nuclear fuelassembly 200, as illustrated by FIG. 2B.

Referring to FIG. 1 again, it may be noted that coolant 150 may serveseveral purposes within nuclear reactor 100. Coolant 160, being heatedby the heat generated from fission reactions, carries away heat from thefuel rods 130 and core 120, and the heat energy of coolant 160 may beconverted to electrical energy. As well, coolant 150, 160 may act as amoderator to slow neutrons to speeds at which they are more likely tosuccessfully react with nuclear fuel material. In a loss of coolantaccident (LOCA), coolant levels may drop within the reactor vessel 105so that heat energy is no longer adequately conveyed out of the reactor,allowing heat to build within the reactor and potentially damage fuelrods 130, including the casing material. Although a loss of coolant mayalso represent a loss of moderator material, and thus result in aslow-down of fission reactions in the nuclear fuel material, heat maystill build rapidly in the reactor vessel as the radioactive nuclearfuel materials, as well as radioactive by-products of fission reactions,continue to radiate heat energy into the reactor. Both the Three MileIsland disaster and the Fukushima Daiichi disaster began as loss ofcoolant accidents, resulting in a meltdown and highly exothermicoxidation of the zirconium alloy cladding, producing vast amounts ofhydrogen gas and resulting in further heat build-up and a subsequentcore meltdown. Once the cladding of fuel rods has been breached orcracked in a meltdown, the radioactive nuclear fuel and its radioactivefission by-products may be exposed and mix with other gases produced bythe meltdown, allowing the radioactive materials to escape into thesurrounding environment.

Incidents such as Three Mile Island have spurred research intoalternative and safer fuel rod cladding materials that can replacezirconium alloy cladding and other cladding materials. Silicon carbide(SiC), for example, may be one such alternative cladding material.Although silicon carbide is a relatively brittle material, itsbrittleness may be mitigated by the use of silicon carbide fiber(SiC_(f)) reinforced silicon carbide matrix (SiC_(m)) Ceramic-MatrixComposite (CMC) structures. FIGS. 3A-3B illustrate one exampleembodiment of a reinforced SiC_(f)—SiC_(m) CMC structure. FIG. 3Adepicts one embodiment of a structure 300 including a tube 310, such asa monolithic SiC tube, with a plurality of reinforcing ribbons 320 ofSiC fibers or tows 340 braided or wound around tube 310. Reinforcingribbons may include, for example, a plurality of SiC fibers or tows 340as illustrated by the close-up view of portion 330 of one ribbon 320.Fibers or tows 340 may include a silicon-carbide compound, such asSiC_(f). FIG. 3A illustrates one example of a braiding or windingprocess and pattern of ribbons 320, with additional alternating strandsnot included in order to simplify the figure and better illustrate theexemplary pattern. Other patterns and processes of braiding or windingribbons may also be possible. For example, ribbons 320 could also bebraided inside tube 310 (not shown in FIG. 3A for clarity ofillustration).

FIG. 3B illustrates structure 300 with multiple layers of ribbons 320encasing tube 310, and embedded an outer layer 360 covering the ribbons320 and tube 310. The roles of 310 and 360 may be reversed, in whichcase 360 is an outer tube encasing the multiple layers of ribbon 320 andthen covered with an inner layer 310. For clarity of presentation, theformer architecture is assumed without loss of generality. Outer layer360 may also include SiC, in which case structure 300 may be aSiC_(f)—SiC_(m) CMC structure. Outer layer 360 may be provided, forexample, by a chemical vapor infiltration (CVI) process and/or achemical vapor deposition (CVD) process. Close-up view 351 shows a viewof a portion of the plurality of fibers 340 as seen looking radiallyinto the structure 300, illustrating how fibers 340 may be ideallyarranged in an ideal ‘cross-weave’ type pattern to provide reinforcementto structure 300 while providing open porosities to facilitate CVD orCVI. Close-up views 352 and 353 show a cross-sectional view of acut-away portion of the plurality of fibers 340, illustrating one way inwhich fibers 340 may be ideally arranged to layer over tube 310. Theexemplary structure 300 illustrated by FIG. 3B depicts eight layers ofribbons 320 layered over tube 310 for illustration purposes only; inpractice, many more layers of ribbons 320 may be provided over tube 310for structural reinforcement, or fewer layers of ribbons 320 may beneeded. Alternatively, tube 310 and matrix 360 may be reversed toreflect a winding or braiding of ribbons 320 inside tube 310.

Reinforced CMC structures, such as the exemplary structure 300illustrated by FIGS. 3A-3B, may have a toughness comparable to metals,such as zirconium alloys, but with much greater tolerance for hightemperatures. For example, beta-phase stoichiometric silicon carbide(β-SiC) CMCs retain their strength at a temperature of 1500° C. underirradiation. As well, β-SiC materials may exhibit low oxidation rates athigh temperatures, and may have a relatively low reactivity with nuclearfuel compounds such as uranium dioxide (UO₂). However, even reinforcedCMC structures are not without drawbacks. For example, although SiCcompounds had been identified as possible substitutes for zirconiumalloy cladding when the Fukushima power plants were built, siliconcarbide cladding fuel rods were still expensive to produce and use.Unlike metal alloys, which may be readily welded to seal fuel pelletswithin a metal alloy cladding, SiC materials do not readily fusetogether, making it difficult to fully hermetically seal nuclear fuelpellets within a silicon carbide tube. As well, SiC CMC reinforcedcladding is generally made relatively thick in order to overcome theinherent brittleness of pure silicon carbide; however, metal cladding ofcurrent fuel rods can be made relatively thin compared to SiC CMCcladding. Thus, in order for many SiC CMC clad fuel rods to be used asreplacements for metal alloy clad fuel rods in current nuclear reactors,the SiC CMC cladding would have to be kept to a thickness similar tometal cladding, but at such thicknesses the cladding may not provideadequate structural reinforcement to the fuel rod.

Tristructural-isotropic (TRISO) nuclear fuel may address some of theseshortcomings. TRISO nuclear fuel encapsulates nuclear fuel in multiplespherical layers enclosed in a SiC sphere. The spherical design,however, provides a relatively low ratio of nuclear fuel volumefraction, requiring higher enrichments and more frequent replacement,thereby increasing the burden of storing spent nuclear fuel safely.

As well, not every silicon carbide CMC may be suitable for use ascladding, but those CMCs that are suitable present challenges anddrawbacks as well. For example, one of the few SiC_(f) tow used toreinforce CMC materials currently being used to develop nuclear fuel rodstructures is Hi-Nicalon Type-S(HNS), a commercially available β-SiC_(f)compound that sufficiently approaches stoichiometry and that canwithstand high doses of neutron bombardment during use in a nuclearreactor. However, HNS fibers typically do not form a well-orderedarrangement of continuous fibers as shown in the close-up views 351, 353depicted in FIG. 3B. Instead, FINS fibers tend to twist and tangle,forming clumps of silicon carbide and leaving spaces or voids within thebraided tow structure around tube 310. These problems occur regardlessof the specific process used to form and deposit the HNS fibers, whetherby chemical vapor infiltration (CVI) and/or chemical vapor deposition(CVD), polymer infiltration and pyrolysis (PIP), or melt infiltrationprocesses. This tendency of HNS fibers to tangle and clump may alsoreduce the resulting CMC 300 fiber volume fraction in some portions ofthe braided fiber structure around tube 310, leaving those portions moresusceptible to cracking. Formation of HNS reinforced CMC cladding by amelt infiltration process also tends to form pockets of silicon alongportions of the HNS fibers; as silicon expands once it turns solid, thesilicon pockets become weak points in the CMC that are highlysusceptible to cracking.

Thus, generally stated, disclosed herein is a nuclear fuel structure orcladding structure which includes a plurality of fibers arranged in thenuclear fuel structure and a multilayer fuel region within at least onefiber of the plurality of fibers. The multilayer fuel region includes aninner layer region with a nuclear fuel material, an outer layer regionencasing the nuclear fuel material. As used herein, the term “fiber” canrefer to any elongated structure along which discrete regions may beformed. This may include, but is not limited to, any CMC structure(s),filaments or filamentary structures, and other similar structures of thetype disclosed herein. Filamentary structures may include, for example,structures that curl around a helix or branch out into multiplefilaments or fibers. When used as cladding, the plurality of fibers maycontain no fiber having a multilayer fuel structure. In any instance,the plurality of fibers may also contain elements intended to interactwith nuclear reactions, for example material included as burnablepoisons.

In one embodiment, the plurality of fibers are arranged in the nuclearfuel structure to provide structural reinforcement to at least a portionof the nuclear fuel structure. The nuclear fuel structure may include aninner rod or tube structure, and the plurality of fibers may be wrappedaround the inner rod or tube structure to facilitate providingstructural reinforcement. As one or more of the plurality of fibers mayalso include a multilayer fuel region or regions within the one or morefibers, a CMC tube reinforced with a plurality of fibers may not onlyserve as stand-alone nuclear fuel but may also serve as claddingcontaining the additional nuclear fuel pellets.

In another embodiment, the inner layer region having the nuclear fuelmaterial may be a first inner region, and the structure may also includea second inner layer region below the first inner layer region. Thesecond inner layer region may include a material selected to captureby-products, such as gaseous by-products, of nuclear fission reactionsoccurring in the nuclear fuel material. As exemplified in FIG. 11, thematerial of the second inner layer region 1102 may be, in one example,nanoporous carbon deposited upon a scaffold filament 1101.

In yet another embodiment, the multilayer fuel region is one multilayerfuel region of a plurality of discrete multilayer fuel regions disposedalong the at least one fiber. The plurality of discrete multilayer fuelregions may each have a respective inner layer region of nuclear fuelmaterial and a respective outer layer region encasing the nuclear fuelmaterial. The plurality of discrete multilayer fuel regions may beformed over a core filament along the length of the at least one fiber.

In yet another embodiment, the fibers may include, in addition to orinstead of a multilayer fuel region, an additional material layerselected to interact with nuclear fuel material in order to moderate ordelay nuclear fission. In one example the additional material layer mayinclude carbon as a moderator. In another example the additionalmaterial layer may include boron or gadolinium as a nuclear poison orburnable poison to delay nuclear fission.

In another aspect, also disclosed herein is a method of facilitatingfabricating a nuclear fuel structure, where the facilitating fabricatingincludes providing a plurality of fibers arranged in the nuclear fuelstructure and forming a multilayer fuel region within at least one fiberof the plurality of fibers. The multilayer fuel region formed includesan inner layer region having a nuclear fuel material, and an outer layerregion encasing the nuclear fuel material. In one example, forming atleast one layer of the multilayer fuel region may be accomplished by achemical vapor deposition (CVD) process. In another example, the methodmay also include providing an inner rod or tube structure of the nuclearfuel structure, and arranging the plurality of fibers to wrap around theinner rod or tube structure so that the plurality of fibers providestructural reinforcement to the nuclear fuel structure.

FIG. 4A depicts one embodiment of a fiber 410 that includes a multilayerfuel region 420. A cutaway view 415 of multilayer fuel region 420 isprovided to show clearly the multiple layers of multilayer fuel region420. Multilayer fuel region 420 has an inner layer region 423 thatincludes nuclear fuel material, such as uranium or a uranium compound,and an outer layer region 424 that encases the nuclear fuel material ofinner layer region 423. The nuclear fuel material of inner layer region423 may be uranium dioxide (UO₂), as uranium dioxide may be usedfrequently as a nuclear fuel in many nuclear fuel structures. However,because the inner layer region 423 is completely, hermetically sealed byouter layer region 424, uranium, plutonium or fissilematerial-containing compounds with a higher fissile material densitythan that of uranium dioxide may also be used, such as uraniummononitride (UN), uranium carbide (UC), and uranium silicide (U₂Si₃).Outer layer region 424 may include, for example, pyrolytic carbon (PyC),and/or may include silicon carbide. In the exemplary multilayer fuelregion depicted in FIG. 4A, inner layer region 423 may be considered afirst inner layer region 423 and multilayer fuel region 420 may includea second inner layer region 422 disposed below the first inner layerregion 423. The second inner layer region may include a material, suchas nanoporous carbon, selected to capture by-products of nuclear fissionreactions, such as gaseous by-products, occurring in the nuclear fuelmaterial of inner layer region 423.

Outer layer region 424 may include, for example, pyrolytic carbon (PyC),and/or may include silicon carbide. Outer layer region 424 may, in oneembodiment, be a first outer layer region 424, and multilayer fuelregion 420 may include a second outer layer region. Second outer layerregion may include a material that adds further functionality to fiber410. For example, a second outer layer region may include a nuclearpoison material, such as boron or gadolinium, that may delay activity ofthe nuclear fuel material of inner layer region 423. Second outer layerregion, in another example, may include a material, such as carbon, thatacts as a moderator of nuclear fission activity of the nuclear fuelmaterial of inner layer region 423. In another example, second outerlayer region may include an interphase layer for structural integrationwith a SiC matrix.

Multilayer fuel region 420 may be formed over a core region 421. Coreregion 421 may be, for example, a silicon carbide filament along alength of fiber 410, over a portion of which the multiple layers ofmultilayer fuel region 420 are formed. Core region 421 generally mayinclude any ceramic material, such as silicon carbide or hafniumcarbide. Finally, an overcoat layer 425 may further encase the multiplelayers of multilayer fuel region 420 and core region 421. Overcoat layer425 may itself be a multilayer overcoat. Overcoat layer 425 may includea ceramic material, such as silicon carbide or hafnium carbide, and mayinclude additional overcoat layers that further functionalize the fiber.For example, an additional layer may be a layer of pyrolytic carbon(PyC) applied as a CMC interphase layer. In another example, anadditional layer may include boron nitride applied to serve both as aninterphase layer and a burnable nuclear poison layer to inhibit nuclearfission reactions in nuclear fuel material 423. Multilayer fuel region420 may be one multilayer fuel region of a plurality of discretemultilayer fuel regions 420 disposed along fiber 410, as illustrated inFIG. 4A. Each discrete multilayer fuel region 420 may have a respectiveinner layer region 423 made of the nuclear fuel material, and arespective outer layer region 424 encasing the nuclear fuel material.The plurality of discrete multilayer fuel regions may be disposed overcore region 421 along the length of the at least one fiber 410. Overcoatlayer 425 may encase the plurality of discrete multilayer fuel regions420 and core filament 421, resulting in a completed fiber 410. Overcoatlayer 425, as depicted in FIG. 4A, may have a substantially uniformthickness along the length of fiber 410. As illustrated by FIG. 4A, theplurality of discrete multilayer fuel regions 420 appear as “beads”disposed along fiber 410, as the plurality of discrete multilayer fuelregions have a greater thickness than regions of fiber 410 includingcore filament 421 and overcoat 425 without a multilayer fuel region.

FIG. 4B depicts an alternative embodiment of a fiber 450 that includes amultilayer fuel region 460, in which the fiber 450 has a substantiallyuniform thickness, so that multilayer fuel region 460 appears to beembedded within fiber 450. In the alternative embodiment of FIG. 4B,first inner layer region 463, second inner layer region 462, outer layerregion 464, and overcoat 465 may have varying thicknesses over a lengthof multilayer fuel region 460, with a thickness of a layer tapering ateither end of multilayer fuel region 460. In one exemplary embodiment,core region 461 may be provided to have a variable thickness prior toproviding the layers of multilayer fuel region 460. In another exemplaryembodiment, core region 461 may have a substantially uniform thickness,and multilayer fuel region 460 may be formed over core region 461 andmay initially have a beaded appearance as depicted in FIG. 4A, butovercoat layer 465 may be provided with a variable thickness overmultilayer fuel region 460 and core region 461, resulting in fiber 450having a substantially uniform thickness along a length of fiber 450.

The exemplary embodiments of fibers 410 and 450, as depicted in FIGS. 4Aand 4B, are only two possible embodiments of a fiber including amultilayer fuel region 420, 460, and including a plurality of discretemultilayer fuel regions 420, 460. Variations of the examples depicted,as well as alternative multilayer fuel region embodiments, may bepossible and contemplated within the scope of the disclosure herein. Forexample, fibers such as fibers 410/450 may have other functionalityadded by either or both of coating or doping. Specifically, fibers mayembed materials intended to either enhance or inhibit nuclear fissionreactions, either temporarily or permanently. In one example, fibers canbe coated or doped with materials intended to enhance nuclear reaction,which transmute into isotopes that are themselves fissionable. Examplesof such isotopes are Thorium-232 and Uranium-238. In another example,fibers can be coated or doped with neutron-absorbing isotopes thatinhibit nuclear reactions. Among such isotopes—known as “nuclearpoisons”—are temporary inhibitors called “burnable poisons”, such asboron and gadolinium. Other elements are long-term inhibitors, such ashafnium. In one example, dopant may be added to a fiber or fibers duringfabrication, and become part of a microstructure of the fiber, either asinterstitial elements, substitution elements, or concentrated at grainboundaries. In yet another example, functional coatings can be coatedlocally over the entire length of a fiber. A coating can also be appliedto the fibers in compound form, where the coating can serve to meetadditional functional requirements. For example, boron can be added inthe form of boron nitride, which can also serve as a lubricantinterphase in a ceramic matrix composite. Boron can also be added asboron carbide. Similarly, hafnium can be coated as hafnium carbide andserve as a high-temperature environmental barrier. Without any loss ofgenerality, any references herein to a fiber 410 and/or multilayer fuelregion 420, as depicted in FIG. 4A, may also be considered to beapplicable to a fiber 450 and/or multilayer fuel region 460, as depictedin FIG. 4B, and vice versa.

FIG. 4C depicts a radial cross-section view through multilayer fuelregion 460 of fiber 450 of FIG. 4B, illustrating exemplary thicknessesof the different layers of multilayer fuel region 460. A similarcross-section view and exemplary thicknesses may apply to multilayerfuel region 420 of FIG. 4A. Core region 461 may be a ceramic materialfilament, such as a silicon carbide filament, with a thickness 461 a ina range of about 5 μm to about 10 μm measured radially from the centerof core region 461 (resulting in core region 461 having a diameterranging from about 10 μm to about 20 μm). First inner layer region 463,having the nuclear fuel material, may have a thickness 463 a rangingfrom about 3 μm to about 30 μm or more. Second inner layer region 462,disposed between the nuclear fuel material 463 and the core region 461,may have a thickness 462 a ranging from about 0.5 μm to about 1.5 μm ormore. Outer layer region 424, encasing the nuclear fuel material offirst inner layer region 463, may have a thickness 464 a ranging fromabout 1 μm to about 2 μm. Overcoat layer 465 over multilayer fuel region460 may have a thickness ranging from about 1 μm to about 2 μm or more,if desired. Overcoat layer 465 may have a similar thickness overportions of fiber 450 that do not have a multilayer fuel region (i.e.,portions of the fiber 450 that include portions of core region 461 andovercoat layer 465 applied over core region 461), or may have a greaterthickness over such portions of fiber 450. Multilayer fuel region 460may thus have an overall thickness 460 a ranging from about 10 μm toabout 22 μm or more, depending on the thicknesses selected for thelayers of multilayer fuel region 460, as measured radially from thecenter of core region 461 to the outer surface of overcoat layer 465.

FIG. 4D may illustrate one embodiment of a portion of multiple fibers410, for example multiple scaffold fibers 410 as illustrated, forexample, in FIG. 10, and may also illustrate multiple fibers 410including pluralities of discrete multilayer fuel regions 420, arrayedto form a ribbon or tow that may be wrapped around an inner rodstructure of a nuclear fuel structure, as further discussed below andillustrated further in FIG. 5A. For clarity the fiber 410 and multilayerfuel region 420 illustrated in FIG. 4A is shown in FIG. 4D to illustrateclearly the plurality of discrete multilayer fuel regions 420 disposedalong fibers 410, with the plurality of discrete multilayer fuel regions420 separated by non-fuel portions 430 of fibers 410 that do not containmultilayer fuel regions, although it is to be understood that multiplefibers 450 as illustrated in FIG. 4B may similarly be arrayed in asimilar ribbon or tow. In exemplary embodiments in which the pluralityof discrete multilayer fuel regions 420 are disposed substantiallyuniformly over a length of fiber 410, any one of the plurality ofdiscrete multilayer fuel regions 420 may, for example, be about 5 mmlong, and any one non-fuel portion 430 may, for example, be about 5 mmlong. The plurality of discrete multilayer fuel regions 420 may thuscover about half or 50% of an overall length of one fiber of themultiple fibers 410. The length dimensions provided are by way ofexample only, as the plurality of discrete multilayer fuel regions maybe formed to have greater or smaller lengths, and may be separated bylarger or smaller non-fuel portions 430 along fiber 410. For example,multilayer fuel regions 420 may be formed to a length of about 6.5 mm,and the non-fuel portions 430 separating the plurality of discretemultilayer fuel regions 420 may be about 3.5 mm in length. In thisexample, the plurality of discrete multilayer fuel regions 420 may coverabout 65% or more of a length of fiber 410.

FIG. 5A depicts one embodiment of a nuclear fuel structure 500 orcladding structure 500 with a plurality of fibers 410/450, such as inthe examples of FIGS. 4A-4B, arranged within nuclear fuel structure 500or cladding structure 500. Nuclear fuel structure 500 has an inner rodor tube structure 520 and an outer layer 560, similar to structure 300of FIG. 3B, and the plurality of fibers 410/450 may be arranged to wraparound inner rod or tube structure 520 to facilitate providingstructural support to nuclear fuel structure 500. Similar to structure300 of FIG. 3B, the respective roles of tube 520 and outer layer 560 canbe swapped, in which case the plurality of filaments 410/450 are woundinside an outer tube 560 and covered with an inner layer 520 to providestructural support to nuclear fuel structure 500. For clarity andsimplicity, the former architecture of inner rod or tube 530 and outerlayer 560 is assumed herein below without loss of generality. Hence,multiple fibers 410/450 of the plurality of fibers include pluralitiesof discrete multilayer fuel regions 420/460, similar to multilayer fuelregions 420/460 of FIGS. 4A-4D, as illustrated more clearly in close-upcross-sectional views 530 and 550 of a portion of the plurality offibers 410/450. Thus, the plurality of fibers 410/450 arranged innuclear fuel structure 500 or cladding structure 500 may provide bothcladding for nuclear fuel pellets and/or provide the nuclear fuelmaterial of nuclear fuel structure 500 and structural reinforcement, orcladding, for nuclear fuel structure 500. The close-up cross-sectionalviews 530 and 550 show one possible arrangement of segments of fibers430 without fuel regions alternating with segments of fibers thatinclude multilayer fuel regions 460, as the fibers might appear if onewere to cut longitudinally along the fibers wrapped around inner rodstructure 520. It may be noted, however, that the alternating patterndepicted in FIG. 5A may not result everywhere in nuclear fuel structure500, as fibers 410/450 need not be wrapped around inner rod structure520 so as to produce such a symmetrical, alternating pattern ofmultilayer fuel regions 460 with non-fuel regions. In practice, across-sectional view 530, 550 of fibers 410/450 might have a randompattern of multilayer fuel regions 460 arrayed with non-fuel regions430. The exemplary nuclear fuel structure 500 illustrated by FIG. 5Adepicts eight layers of fibers 410/450 layered over inner rod or tubestructure 420 for illustrative purposes only, and it may be understoodthat many more layers of fibers 410/450 including multilayer fuelregions may be provided to provide more nuclear fuel within nuclear fuelstructure 500 and provide greater structural reinforcement to nuclearfuel structure 500.

Referencing FIGS. 4A-4D and 5A again, fiber 410 or fiber 450 of FIGS.4A-4B may provide a greater volume of nuclear fuel material for nuclearfuel structure 500 than a volume of nuclear fuel material possible fornuclear fuel rods in current use. The volume of nuclear fuel materialthat can be packed into nuclear fuel structure 500 may be a matter ofvolume fraction of the fiber 410/450 that is nuclear fuel, and a volumefraction of fiber 410/450 taken up by the composite (CMC) materials.These are respectively obtained from equations EQ. 1 and EQ. 2 below,where:

-   -   u_(ff) and f_(f) are respectively the fuel volume fraction of        the fiber, and the fiber volume fraction of the composite,    -   d_(c) and f are respectively the fiber core and outer diameters,        and d and D the nuclear fuel structure 500 inner and outer        diameters,    -   t_(n) and t_(f) are the respective thicknesses of the nanoporous        carbon and fuel layers,    -   δ_(i) and δ_(o) are the nuclear fuel structure's respective        inner and outer layers of monolithic SiC thicknesses,    -   n is the number of layers in the braid,    -   c is the fraction of fiber length covered by fuel cells, and    -   p is the pitch distance between adjacent filaments in a layer.

$\begin{matrix}{u_{ff} = {4c\frac{\left( {d_{c} + t_{n} + t_{f}} \right)}{f^{2}}t_{f}}} & \left( {{EQ}.\mspace{11mu} 1} \right) \\{f_{f} = \frac{n \cdot \pi \cdot \left( {d + \delta_{i} + \delta_{o} + {n \cdot f}} \right)}{\left( {D^{2} - d^{2}} \right)p}} & \left( {{EQ}.\mspace{11mu} 2} \right)\end{matrix}$

For example, consider a fuel embedded in a 30 μm fiber, as shown in theexample of FIG. 4C. If it is assumed that the fuel cells to cover 65% ofthe fiber's length, the volume fraction of the filament occupied bynuclear fuel is 33%. This is over 2.5 times the fuel packing density ofTRISO. Similar to TRISO fuel, because the fuel is fully sealed in SiC,the fissile material content can be nearly doubled compared to UO2 byembedding UC, UN, U₂Si₃ or even U as nuclear fuel.

Referring again to FIGS. 4A-4D and 5A, the issue of fiber packingdensity using the fibers described herein may be examined. Table 1compares alternate designs for fiber volume fraction, and against TRISOfor fuel volume fraction for various designs of tube inner diameter(‘ID’) and outer diameter (‘OD’). The analysis also assumes inner andouter monolithic SiC layer 500 μm thick sealing in the CMC tube, and anintra-layer pitch of 40 μm center to center between filaments. Althoughthe embodiments described so far have assumed a tube configuration,alternative embodiments may include braiding over a monolithic SiC rod,which is what design No. 3 in Table 1 represents.

Table 1 shows the superior fiber packing density afforded by the ribbonarchitecture introduced in FIGS. 3A-3B and 5A, comparing alternateembodiments or designs for fiber volume fraction and fuel volumefraction. Indeed, ceramic tow weaving or braiding seldom can producefiber volume fractions reaching 30%, which is important for thestructural strength and toughness of the CMC. The higher fiber volumefraction is achieved without exacerbating the “labyrinth effect” whichtypically prevents adequate infiltration by the matrix and results inunwanted voids in the CMC. The implementation suggested by the examplestreated in would leave a well-controlled pore distribution of 10 μmbetween filaments, allowing for even diffusion of the matrix precursorsthroughout the volume of the CMC.

More importantly for nuclear energy applications, the proposed approachallows fuel packing densities that are up to 3 times as much as TRISOspherical fuel, with the added benefit—assuming a tube—that heat couldbe convectively extracted from both the inner and outer surfaces, henceenhancing heat transfer. A final, but important remark, is thatembodiments including tube designs, such as tube designs 1 and 3 inTable 1 below, could be made as drop-in replacements for Zircaloy fuelrods in light water reactors (LWR).

TABLE 1 Comparing alternate embodiments of a nuclear fuel structure forfiber volume fraction and fuel volume fraction. ID OD Fiber volume Fuelvolume # (in.) (in.) Layers fraction fraction Reference TRISO  6.5% 1 ¼½ 72  32% 10.5% 2 ½ 1 178 49.5% 16.3% 3 ⅛ ½ 125 46.3% 15.25% 

Referring to FIGS. 5A-5E, through the use of a nuclear fuel structuresuch as nuclear fuel structure 500 it may be possible to achieve a fuelassembly design for which a fuel volume fraction exceeds an annulusminimum areal fuel load q of about 0.443576. Achieving such a fuelassembly design may be characterized as a paving problem, in which thepaving problem may be parameterized as described below.

For example, as depicted in part by FIG. 5B, it may be assumed that anintegral fuel tube will be in a square pattern with a center-to-centerdistance m. The inner and outer diameters of the integral fuel tube ared and D respectively and the areal fuel loading in the tubecross-section is q. Further ρ and μ designate respectively:

$\begin{matrix}{\rho = \frac{D}{m}} & \left( {{EQ}.\mspace{11mu} 3} \right) \\{µ = \frac{d}{D}} & \left( {{EQ}.\mspace{11mu} 4} \right)\end{matrix}$

The paving problem may be reduced to a single tile, with an areal fuelload given by EQ. 5 below:

UVF _(t)=(π/4)qρ ²(1μ²)  (EQ. 5)

EQ. 5 governs the design space of feasible solutions for a fuelassembly. Assuming the fuel assembly is paved with such tiles, then thetile's areal fuel load is the same as that of the FA. FIG. 5B depicts anexample cross-section of a fuel assembly 501 including fuel rods 560.For the example 5×5 grid of distributed over a 214 mm×214 mmcross-section of fuel assembly 501, as depicted in FIG. 5B, acenter-to-center distance 563 m=42.8 mm. As well, for the example fuelassembly 501, an inner to outer tube diameter ratio μ≈⅓, so that thetube's wall thickness is of the order of the inner diameter. As anillustrative example, we pick an annulus ID 562 and OD 561 respectivelyat 12.6 mm and 41.9 mm, i.e. ρ=0.978 and μ=0.3.

The tables below show two sample design configurations that exceed therequired annulus areal fuel load of q=0.443576. The designs differ onlyin their constitutive fibers and the corresponding monolithic layer. Thefuel assembly areal fuel loads for these designs are 30.88% and 30.44%respectively. Both are greater than a benchmark areal fuel load of30.36%.

TABLE 2 Comparing alternate design configurations of fuel assemblyhaving annulus arreal fuel load q > 0.443576.

As the share of fuel assembly cross-section functionally allocated tofuel is increased, the share of cross-section allocated to coolant flowmay be reduced compared to other fuel assembly designs. The totalconvective perimeter may also be reduced to 4.28 m, a 45% reductioncompared to other designs. This may require a two-fold improvement ofconvective heat transfer, which can be achieved with a flow increase, anincreased operating temperature, or a combination thereof. Fortunatelyin this case, higher operating temperatures are not only permitted bythe material, they are also desirable for thermal efficiency. It is alsoworth noting that with current fuel pellet-based design, conductive heattransfer is a limiting factor due to the poor thermal conductivity ofUO₂. Conductive heat transfer is no longer limiting in the case of CMCcontaining fuel in fibers as conductivity is increased by about twoorders of magnitude by the SiC matrix and fibers.

FIGS. 5C-5E depict cross-sectional views of alternative design variantsof the fuel assembly depicted in FIG. 5B. The alternative designsdepicted by FIGS. 5C-5E may have a similar fuel assembly areal contentwith different convective perimeters.

Referring again to FIG. 5A, any of the described embodiments of nuclearfuel structure 500, as well as alternative embodiments, may provideseveral additional benefits in addition to those described above. Infuel rods currently in use, a structural breach in the cladding orcasing may risk exposing a large amount of the nuclear fuel contained inthe fuel rod, and potentially may expose all of the radioactive nuclearfuel to the surrounding environment. Because the nuclear fuel materialof nuclear fuel structure 500 is embedded inside the plurality of fibers410/450 in a plurality of discrete and separated multilayer fuelregions, rather than being deployed inside a tube, any breach in thestructure of nuclear fuel structure 500 may only expose a small amountof the total nuclear fuel material, minimizing the amount of hazardousradioactive material that may escape into the surrounding environment inthe event of an accident. As well, it may be extremely difficult andextremely dangerous for nuclear fuel material to be recovered fromnuclear fuel structure 500 for misuse in making weapons, as the nuclearfuel material in inner layer regions 423 is embedded within carbon andsilicon carbide materials and formed over carbon and silicon carbidelayers as well. Finally, because nuclear fuel is embedded and sealedwithin the plurality of fibers before the fibers are wrapped aroundinner rod structure 420 of the nuclear fuel structure 500, there may beno need to provide a cap to seal nuclear fuel structure 500. This mayeliminate problems with trying to fuse a silicon carbide sealing cap toends of a silicon carbide fuel structure or cladding.

As well, pure stoichiometric β-SiC fibers are capable of being resistantfor long periods of time (>8 years) in close proximity to nuclear fuel.To date, the only SiC fibers to have achieved the required stoichiometryand purity have been deposited by CVD on a tungsten or carbon corefilament. Such fibers, however, come only in large diameters (90 or 140μm) that are unsuitable for the kind of braiding or weaving as disclosedherein, let alone the presence of a foreign core filament that would notnecessarily survive the nuclear reactor environment.

As discussed previously, a source of SiC_(f) tows approachingstoichiometry and purity is HNS. There are two issues of critical importassociated with HNS: Composition, and foreign sourcing. HNS fibers areproduced by spinning a preceramic polymer, which must then be processedat great expense of money and energy to reduce impurity levels. Theseextreme levels of processing drive the cost of HNS to roughly$10,000/lb. yet only reduce oxygen contents down to 0.2-0.7%, which isbarely acceptable for long-lived nuclear applications. The limitation onoxygen content is inherent to chemical processes that only achievepurity in the limit. It is therefore likely to recur with any preceramicpolymer approach to SiC.

The issue of foreign sourcing has also long been a frustration to the USgovernment and industry. As disclosed herein, a CVD process is capableof producing a wide range of filament diameters (10-100 μm), without therequirement of a core filament. The nuclear fuel structures andprocesses for making such may include printing 3C-βSiC filaments thatexhibit stoichiometry and purity in a single step, and that may notrequire foreign sourcing.

Referring again to FIG. 5A, in one alternative embodiment the pluralityof fibers 410/450 may also include multiple sensor fibers. The multiplesensor fibers may be arranged with the multiple fibers including theplurality of discrete multilayer fuel regions. Sensor fibers mayinclude, for example, silicon carbide filaments coated with zirconiumdiboride (ZrB₂), and may include, as another example, silicon carbidefilaments coated with hafnium diboride (HfB₂). In one embodiment,multiple zirconium diboride coated fibers may be braided with multiplehafnium diboride coated fibers, wherein each overlap or contact pointbetween a zirconium diboride fiber and a hafnium diboride fiber providesa high temperature thermocouple. The resulting braid would form a squarematrix of embedded thermal receptors capable of mapping temperaturethroughout the structure. In exemplary embodiments the boron of thezirconium diboride and hafnium diboride includes the ¹¹B isotope toensure that the fibers may be compatible with nuclear reactors.

FIGS. 6A-6E depict one example of a part of a process for forming anuclear fuel structure, such as nuclear fuel structure 500, includingforming a multilayer fuel region within at least one fiber of aplurality of fibers. The process depicted in FIGS. 6A-6E may bedescribed as forming a multilayer fuel region by spot-coating, ordepositing a layer of material of a specified thickness over a givenlength of the at least one fiber. FIG. 6A depicts a portion of a fiber600 including a core filament 610. Core filament 610 may be a coreregion, as described above, and may include a ceramic material such assilicon carbide or hafnium carbide. In the example depicted in FIG. 6A,core filament 610 may have a substantially uniform thickness.

FIG. 6B depicts core filament 610 of FIG. 6A having a material layer 620deposited over a portion of core filament 610, where material layer 620includes a material selected to absorb gaseous by-products of nuclearfission reactions occurring in a nuclear fuel material. Material layer620 may correspond to a second inner layer region 422 depicted in theexample of FIG. 4A. In exemplary embodiments material layer 620 mayinclude nanoporous carbon. A material layer 620 of nanoporous carbon maybe provided, for example, by chlorine etching of a part of core filament610. Alternatively, a material layer 620 of nanoporous carbon may bespot-coated onto core filament 610.

FIG. 6C depicts fiber 600 of FIG. 6B with a nuclear fuel material 630deposited over at least a part of a length of material layer 620.Nuclear fuel material may include one or more fissile materials such asuranium, plutonium and/or related compounds, for example uraniumdioxide, uranium mononitride, uranium carbide, and/or uranium silicide.Nuclear fuel material 630 may be provided, for example, by an LCVDprocess using, for example, uranium hexafluoride (UF₆) as a precursorfor forming the nuclear fuel material layer. Alternatively, uraniumhexafluoride (UF₆) may be used as a precursor for LCVD along withappropriate precursors such as ammonia, methane or chorosilane for theformation of a UN, UC or U₂Si₃ layer 630. Nuclear fuel material 630 inFIG. 6C may correspond to a first inner layer region 423 of FIG. 4A.

FIG. 6D depicts fiber 600 of FIG. 6C with an outer layer region 640deposited over nuclear fuel material 630 and material layer 620 of FIG.4C. In exemplary embodiments, outer layer region 640 encases nuclearfuel material 630 to seal the nuclear fuel within fiber 600. Outer layerregion 640 may include, for example, pyrolytic carbon deposited by, inone instance, an LCVD process. Outer layer region 640 and inner layerregion 630 including the nuclear fuel material, at least, form amultilayer fuel region of fiber 600.

In one embodiment, outer layer region 640 may be a first outer layerregion 640, and a second outer layer region may be included. Secondouter layer region may be added to add further functionality to fiber600. For example, a second outer layer region may include a nuclearpoison material, such as boron or gadolinium, that may delay activity ofthe nuclear fuel material of inner layer region 630. Second outer layerregion, in another example, may include a material, such as carbon, thatacts as a moderator of nuclear fission activity of the nuclear fuelmaterial 630. In another example, second outer layer region may includean interphase layer for structural integration with a SiC matrix.

FIG. 6E depicts fiber 600 of FIG. 6D with an overcoat layer 650deposited over fiber 600, covering both core filament 610 and themultilayer fuel region. Overcoat layer 650 may be provided, for example,by an LCVD process. In the example embodiment illustrated by FIG. 6E,overcoat layer 650 may have a substantially uniform thickness over fiber600, resulting in the multilayer fuel region having a “beaded”appearance, as shown. In an alternative embodiment, overcoat layer maybe formed to have a variable thickness over fiber 600, which may resultin greater deposition of overcoat layer 650 over core filament 610 andlesser deposition of overcoat layer 650 over the multilayer fuel region.In such an alternative embodiment, the resulting fiber 600 may have auniform appearance, as depicted in FIG. 7E.

FIGS. 7A-7E depict an alternative embodiment of the process illustratedby FIGS. 6A-6E, in which core filament 710 of fiber 700, as shown inFIG. 7A, has a variable thickness over a length of fiber 700. Forexample, core filament 710 may have a smaller thickness over firstportions 711 of core filament 710, and greater thickness 712 over secondportions 712 of core filament 710. As illustrated in FIGS. 7B-7E, thelayers of a multilayer fuel region may be formed over first portions 711so that the multilayer fuel region, as finally formed, has a thicknesssubstantially equal to the thickness of second portions 712.

FIG. 7B depicts core filament 710 of FIG. 7A having a material layer 720deposited over first portion 711 of core filament 710, where materiallayer 720 includes a material selected to absorb by-products of nuclearfission reactions occurring in a nuclear fuel material. Material layer720 may correspond to second inner layer region 462 as depicted in theexample of FIG. 4B. In exemplary embodiments material layer 720 mayinclude nanoporous carbon. A material layer 720 of nanoporous carbon maybe provided, for example, by chlorine etching of a part of core filament710. Alternatively, a material layer 720 of nanoporous carbon may beprovided by spot-coating.

FIG. 7C depicts fiber 700 of FIG. 7B with a nuclear fuel material 730deposited over material layer 720. Nuclear fuel material may include oneor more fissile materials such as uranium, plutonium and/or relatedcompounds, for example uranium dioxide, uranium mononitride, uraniumcarbide, and/or uranium silicide. Nuclear fuel material 730 may beprovided, for example, by an LCVD process. Nuclear fuel material 630 maybe provided by an LCVD process using, for example, uranium hexafluoride(UF₆) as a precursor for forming the nuclear fuel material layer.Alternatively, uranium hexafluoride (UF₆) may be used as a precursor forLCVD along with appropriate precursors such as ammonia, methane orchorosilane for the formation of a UN, UC or U₂Si₃ layer 730. Nuclearfuel material 730 of FIG. 7C may correspond to inner layer region 463 ofFIG. 4B.

FIG. 7D depicts fiber 700 of FIG. 7C with an outer layer region 740deposited over nuclear fuel material 730 and material layer 720 of FIG.7C. In exemplary embodiments, outer layer region 740 encases nuclearfuel material 730 to seal the nuclear fuel within fiber 700. Outer layerregion may include, for example, pyrolytic carbon deposited by, in oneinstance, an LCVD process. Outer layer region 740 and inner layer region730 including the nuclear fuel material, at least, form a multilayerfuel region of fiber 700. Multilayer fuel region of fiber 700 may nowhave a thickness substantially equal to the thickness of second portions712 of core filament 710.

In one embodiment, outer layer region 740 may be a first outer layerregion 740, and a second outer layer region may be included. Secondouter layer region may be added to add further functionality to fiber700. For example, a second outer layer region may include a nuclearpoison material, such as boron or gadolinium, that may delay activity ofthe nuclear fuel material of inner layer region 730. Second outer layerregion, in another example, may include a material, such as carbon, thatacts as a moderator of nuclear fission activity of the nuclear fuelmaterial 730. In another example, second outer layer region may includean interphase layer for structural integration with a SiC matrix.

FIG. 7E depicts fiber 700 of FIG. 7D with an overcoat layer 750deposited over fiber 700, covering both core filament 710 and themultilayer fuel region. Overcoat layer 750 may be provided, for example,by an LCVD process. The resulting fiber 700 may have a substantiallyuniform thickness over a length of fiber 700 following provision ofovercoat layer 750. Multilayer fuel region of fiber 700 may thus beembedded within fiber 700.

The embodiments of the processes depicted in FIGS. 6A-6E and FIGS. 7A-7Emay not only be applied to one fiber, but may be applied to multiplefibers arrayed together in a ribbon or tow-like structure, so that eachlayer of a multilayer fuel region for one fiber is also formed over theother multiple fibers, as shown in FIG. 8. Each step of layer formationmay be carried out in a separate deposition tool, an example of which isdepicted in FIG. 8, and the multiple fibers may be conveyed from onedeposition tool to the next for the next layer to be deposited. As well,the deposition tool or tools may be controlled to automatically stop andstart deposition of layers over the multiple fibers, thus allowing for aplurality of discrete multilayer fuel regions to be formed along thelengths of the multiple fibers while also automatically forming non-fuelregions of the fiber that separate the plurality of discrete multilayerfuel regions.

FIG. 8 depicts one example of a deposition tool 800 that may be used toform a layer of a multilayer fuel region of at least one fiber, orrespective layers of respective multilayer fuel regions for a pluralityof fibers. Deposition tool 800 may, for example, be a laser chemicalvapor deposition (LCVD) tool. Deposition tool 800 may convey multiplefibers 830 through a conveyer inlet 815 into a deposition chamber 830.Deposition chamber may contain one or more precursor gases that mayfacilitate forming a layer of a multilayer fuel region. A laser 820 maybe provided, through a focusing lens or window 825, to be incident onmultiple fibers 840 as the multiple fibers 840 are conveyed through thedeposition chamber. As the laser 820 interacts with the multiple fibers840 and precursor gases, the desired layer of a multilayer fuel regionmay be deposited over portions of the multiple fibers 845. In oneexample, the laser may be started and stopped at defined intervals asthe multiple fibers pass through the deposition tool 800, thuscontrolling formation of multilayer fuel regions over portions of themultiple fibers 845 and leaving other portions unprocessed (i.e.,non-fuel regions of the multiple fibers). The processed multiple fibers845 may then be conveyed out of the deposition tool 800. The multiplefibers 845 may then be conveyed to another deposition tool, in whichanother layer of the discrete multilayer fuel regions will be formed, ormay be finished and conveyed out of the tool entirely. The resultingmultiple fibers may then be further arranged in a nuclear fuelstructure, such as nuclear fuel structure 500, to be wrapped around aninner rod structure, as described herein. For clarity, FIG. 8 includesclose-up views 810 and 815 of the multiple fibers 840, 845 as themultiple fibers undergo LCVD processing to deposit a layer of themultilayer fuel regions.

FIG. 9 depicts one embodiment of a process 900 for forming a pluralityof fibers arranged in a lattice 910. Magnified views 901 and 902 depicta filament lattice 910 including a plurality of filaments 920 undergoingtreatment by a plurality of laser beams 930 in a LCVD process. Pluralityof laser beams 930 induce a plasma 940 around a tip of the plurality offilaments 920, adding material to the plurality of filaments 920 to formthe plurality of fibers. The plurality of fibers may, in turn, be theplurality of fibers depicted in any of FIGS. 3A-8, as described above.The LCVD process of FIG. 9 may, in one example, be controlled to form aplurality of fibers having a substantially uniform thickness. In anotherexample, the LCVD process of FIG. 9 may be variably controlled to form aplurality of fibers having variable thickness along the lengths of theplurality of fibers. For example, the plurality of laser beams 930 mayhave an intensity that may be increased or decreased as the plurality offibers are formed, resulting in corresponding increases or decreases inthe amount of material added to the plurality of filaments 920 offilament lattice 900. Depicted in FIG. 9 is one exemplary method andapparatus for forming a plurality of fibers from (e.g., CVD) precursors,including a reactor adapted to grow a plurality of individual fibers;and a plurality of independently controllable lasers, each laser of theplurality of lasers growing a respective fiber of the plurality offibers. The reactor and lasers may grow the fibers according to LaserInduced Chemical Vapor Deposition. The plurality of lasers in oneembodiment comprises Quantum Well Intermixing (QWI) lasers. Thistechnique is further discussed in PCT Publication WO2013180764 (A1)dated 2013 Dec. 5, entitled “HIGH STRENGTH CERAMIC FIBERS AND METHODS OFFABRICATION”, filed as PCT Application WO2013US22053 20130118; and thefollowing three (3) previously filed U.S. Provisional PatentApplications: U.S. Provisional Application No. 61/588,733, filed Jan.20, 2012, entitled “METHOD AND APPARATUS FOR LARGE SCALE MANUFACTURINGOF HIGH STRENGTH CERAMIC FIBERS USING A PLURALITY OF CONTROLLABLELASERS”; U.S. Provisional Application No. 61/588,765, filed Jan. 20,2012, entitled “NON-BRIDGING IN-SITU BORON NITRIDE COATING OF SILICONCARBIDE FIBERS IN CERAMIC MATRIX COMPOSITE MATERIALS”; and U.S.Provisional Application No. 61/588,788, filed Jan. 20, 2012, entitled“NANOCOATING SYSTEMS FOR HIGH PERFORMANCE FIBERS FOR DIRECTINGMICRO-CRACKS AND ENDOWING MATRIX COMPOSITES WITH AN IMMUNE RESPONSE TOMICRO-CRACKING AND OXIDATION”. Each of the above-noted PCT andprovisional applications is hereby incorporated herein by reference inits entirety.

FIG. 10 depicts an exemplary embodiment of the plurality of filaments ofFIG. 9 in lattice 910 resulting from variation in the laser power oflaser beams 930. The filament section 1001 produced at the highest levelof laser power has the largest thickness. As laser power decreasessmoothly over the section of filament 1002, ending with section 1003. Aslaser power increases back up, so does filament thickness until it maxesout in section 1004.

Alternatively, the plurality of fibers may be formed by using “DigitalSpinneret” (“DS”). This technology may also be known as a ‘Fiber LaserPrinter.’ The DS technology induces the growth of parallel monofilamentsby massive parallelization of Laser Induced Chemical Vapor Deposition(“LCVD”), similar to the technique depicted in FIG. 9, in which laserincidence occurs at a glancing angle to a substrate. One exampleembodiment of a SiC_(f) ribbon 910 that may be produced by this methodis shown in FIG. 10. The resulting filaments may be β-SiC 3C with grainsize distribution varying from the fiber center outward. Grains at theedge of the fiber are equiaxed. The anisotropy of the laser printingprocess manifests itself at the fiber's center where grains areelongated along the fiber's axis, and present an aspect ratio of 2-3 ormore, with a radial size of about 25 nm or more. The grain distributionmay provide additional toughness.

Any one or more of the nuclear fuel structures 500 disclosed herein maynot only be appropriate for use in existing nuclear reactors, and maysubstitute directly for metal alloy cladding fuel rods, but may also beappropriate for use in nuclear thermal propulsion (NTP) applications.Nuclear thermal propulsion (NTP) has been a technical area of interestfor the United States federal government and NASA since the late 1950's.Nuclear fuel structures 500 may offer several advantages for harnessingnuclear fission in a spacecraft engine, and may provide a nuclear fuelstructure design equivalent to a hexagonal fuel element building blockas developed by the Nuclear Engine for Rocket Vehicle Applications(NERVA) program. The following NERVA engineering parameters may beapplied to determine a fuel-in-fiber system using a nuclear fuelstructure such as nuclear fuel structure 500: a) uranium fuel density600 mg/cc; b) hexagonal element leg length 0.753″; c) 19 nozzlesequivalent to 19 channels found in hexagonal element.

The resulting silicon carbide-silicon carbide (SiC—SiC) nozzle geometrywould utilize a notional 2 millimeter (mm) inner diameter/4 mm outerdiameter SiC monolithic tube mandrel and require 2 meters of fibberribbon per inch of tube. The fiber volume fraction from this designwould be 30%. With micro-encapsulated fuel cells covering 50% of thefiber's length, the fiber fuel content would be 13.4% and thusequivalent to the NERVA hexagonal fuel. The ribbon-wound mandrelstructure would subsequently be infiltrated with a SiC matrix by eitherthe chemical vapor infiltration or the polymer impregnation andpyrolosis process. In this manner, the SiC_(f)—SiC_(m) fuel-in-fibercomposite nozzle structure would be fabricated. An interesting variationon the fuel layer composition would be to deposit thorium (²³²Th) as afertile material for subsequent neutron activation and transmutation toa fissile uranium species (²³³U).

There may be multiple advantages of using a nuclear fuel structure 500for deployment in NTP. These may include the following:

-   -   1) Manufacturing ease—the LCVD additive manufacturing approach        can produce a full fuel-in-fiber structure in-situ without the        necessity of additional post-fabrication processing. There are        multiple levels of economic savings possible via this method. An        LCVD deposition system, as disclosed herein, is relatively        straightforward, easily scalable, and is composed of        significantly less expensive equipment than other CVD and        additive manufacturing processes, thus reducing the capital        outlay requirements to establish a high throughput manufacturing        plant. The operational costs for running and maintaining such        fiber production systems are similarly less expensive, including        the outlays for raw materials and consumables.    -   2) Operational temperature range capability—the high purity        materials deposited in the baseline fiber and overcoat layers,        in particular the lack of oxygen and other detrimental        contaminants in the structure, will be able to survive the 2600K        operating temperature requirements for a NTP engine.    -   3) Favorable SiC thermal conductivity—the relatively high        thermal conductivity of SiC enhances the ability to remove the        heat generated by the fission process occurring in the fuel        layers along the fiber length. SiC thermal conductivity values        generally range in the 100-150 Watts/meter-K at room        temperature, falling to 20-30 W/m-K at temperatures greater than        1500 C. The concept of a SiC_(f)—SiC_(m) composite nozzle with        high heat transfer efficiency could find application in the        NERVA NTP engine concept. Fuel-in-fiber wound SiC nozzles would        be located in bored passages through the graphite (or other        material) block in which H₂ propellant travels through the tube        inner diameter and is heated.    -   4) Capture of fission gas by-products—the nano-porous carbon        layer adjacent to the deposited fuel layer in the fuel-in-fiber        design may serve as a tortuous path medium that effectively        traps the fission gas by-products, thus preventing these        materials' release into the propellant stream.    -   5) Utilization of uranium nitride (UN) fuel—the overall        integrated fuel package of a SiC_(f)—SiC_(m) nozzle would        provide a barrier to exposure of a UN fuel layer to H₂        propellant, thus minimizing the chemical attack and degradation        of this fuel material. The advantages of UN fuel include higher        uranium fuel density, significantly higher melting point        (approaching 3000K at 1 atm) and enhanced thermal conductivity        (approximately 20 W/m-K).    -   6) Safety enhancements—the issue of nuclear fuel safety is        obviously a central concern for implementation of NTP technology        that needs to be addressed to the satisfaction of government        regulators and the general population. Three example safety        considerations are:        -   a. Accident/crash tolerance in which the integrity of the            fuel encapsulation is maintained. The nuclear fuel structure            500 described herein would lead to enhanced protection            because the fuel component is enclosed in a multitude of            physically isolated micro-cells protected by outer coating            layer(s) and embedded in a solid matrix. Should a fracture            develop, only a minute fraction of the cells can be            breached, hence greatly limiting the release of fissile            material in case of a crash.        -   b. From the NERVA program, a major issue arose as fissile            material was ejected into the propellant stream during            testing due to hydrogen gas etching of the graphite block            with UO₂ or UC₂ particles. This chemical attack and material            release would be mitigated due to the structure of the            nozzle as well as having the nozzles embedded in a solid            graphite matrix.        -   c. For nozzles prepared with fertile nuclear material rather            than fissile, the risks of diversion for WMDs is greatly            diminished, and unused tubes will not represent a high-level            nuclear waste.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprise” (andany form of comprise, such as “comprises” and “comprising”), “have” (andany form of have, such as “has” and “having”), “include” (and any formof include, such as “includes” and “including”), and “contain” (and anyform contain, such as “contains” and “containing”) are open-endedlinking verbs. As a result, a method or device that “comprises”, “has”,“includes” or “contains” one or more steps or elements possesses thoseone or more steps or elements, but is not limited to possessing onlythose one or more steps or elements. Likewise, a step of a method or anelement of a device that “comprises”, “has”, “includes” or “contains”one or more features possesses those one or more features, but is notlimited to possessing only those one or more features. Furthermore, adevice or structure that is configured in a certain way is configured inat least that way, but may also be configured in ways that are notlisted.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below, if any, areintended to include any structure, material, or act for performing thefunction in combination with other claimed elements as specificallyclaimed. The description of the present invention has been presented forpurposes of illustration and description, but is not intended to beexhaustive or limited to the invention in the form disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the invention.The embodiment was chosen and described in order to best explain theprinciples of one or more aspects of the invention and the practicalapplication, and to enable others of ordinary skill in the art tounderstand one or more aspects of the invention for various embodimentswith various modifications as are suited to the particular usecontemplated.

What is claimed is:
 1. A nuclear fuel structure comprising: a pluralityof fibers arranged in the structure; a multilayer fuel region within atleast one fiber of the plurality of fibers, the multilayer fuel regioncomprising: an inner layer region comprising a nuclear fuel material;and an outer layer region encasing the nuclear fuel material.
 2. Thestructure of claim 1, wherein the plurality of fibers are arranged inthe nuclear fuel structure to provide structural reinforcement to atleast a portion of the nuclear fuel structure.
 3. The structure of claim2, wherein the nuclear fuel structure comprises an inner rod structure,and wherein the plurality of fibers are arranged to wrap around theinner rod structure and facilitate providing the structuralreinforcement.
 4. The structure of claim 2, wherein the nuclear fuelstructure comprises an outer tube structure and wherein the plurality offibers are arranged to wrap inside the outer tube structure andfacilitate providing the structural reinforcement.
 5. The structure ofclaim 1, wherein the inner layer region comprising the nuclear fuelmaterial is a first inner region, and the structure further comprises asecond inner layer region disposed below the first inner layer region,the second inner layer region comprising a material selected to captureby-products of nuclear fission reactions occurring in the nuclear fuelmaterial.
 6. The structure of claim 5, wherein the second inner layerregion comprises a nanoporous carbon material.
 7. The structure of claim1, wherein the outer layer region encasing the nuclear fuel comprisespyrolytic carbon and/or silicon carbide.
 8. The structure of claim 1,wherein the outer layer region is a first outer layer region, and thestructure further comprises a second outer layer region, the secondouter layer comprising a material selected to inhibit or moderatenuclear fission reactions occurring in the nuclear fuel material.
 9. Thestructure of claim 8, wherein the material of the second outer layerregion inhibits nuclear fission reactions, and the material comprisesone or more of boron, gadolinium, or hafnium.
 10. The structure of claim1, wherein the multilayer fuel region is one multilayer fuel region of aplurality of discrete multilayer fuel regions disposed along the atleast one fiber of the plurality of fibers, the plurality of discretemultilayer fuel regions comprising a respective inner layer regioncomprising the nuclear fuel material and a respective outer layer regionencasing the nuclear fuel material.
 11. The structure of claim 10,further comprising a core region along the length of the at least one ofthe fibers over which the plurality of discrete multilayer fuel regionsare disposed.
 12. The structure of claim 11, wherein the core regioncomprises a ceramic material.
 13. The structure of claim 11, furthercomprising an overcoat layer along the at least one of the fibers, theovercoat layer further encasing the plurality of discrete multilayerfuel regions and the core region.
 14. The structure of claim 13, whereinthe overcoat layer comprises a plurality of overcoat layers
 15. Thestructure of claim 14, wherein at least one additional overcoat layer ofthe plurality of overcoat layers comprises a material selected toinhibit nuclear fission reactions occurring in the nuclear fuelmaterial.
 16. The structure of claim 10, wherein the respective innerlayer regions of the plurality of discrete multilayer layer regionscomprise at least 33% of a volume of at least one fiber of the pluralityof fibers.
 17. The structure of claim 10, wherein the at least one fiberis one fiber of multiple fibers comprising pluralities of discretemultilayer fuel regions, the multiple fibers being at least a subset ofthe plurality of fibers, wherein the multiple fibers comprise at least30% of a volume of the structure, and wherein the respective inner layerregions comprising nuclear fuel material of the multiple fibers compriseat least 10% of the volume of the structure.
 18. The structure of claim1, wherein the nuclear fuel material comprises fissionable material or acompound thereof.
 19. The structure of claim 1, wherein the nuclear fuelmaterial comprises a fissile material having a higher fissile materialconcentration than a fissile material concentration of uranium dioxide.20. The structure of claim 19, wherein the uranium-containing compoundcomprises one or more of uranium, uranium mononitride, uranium carbide,and/or uranium silicide.
 21. A method comprising: facilitatingfabricating a nuclear fuel structure, the facilitating fabricatingcomprising: providing a plurality of fibers arranged in the structure;forming a multilayer fuel region within at least one fiber of theplurality of fibers, the multilayer fuel region comprising: an innerlayer region comprising a nuclear fuel material; and an outer layerregion encasing the nuclear fuel material.
 22. The method of claim 21,wherein the inner layer region comprising the nuclear fuel material is afirst inner region, the multilayer fuel region further comprising asecond inner layer region formed prior to forming the first inner layerregion, the second inner layer region comprising a material selected tocapture by-products of nuclear fission reactions occurring in thenuclear fuel material.
 23. The method of claim 21, wherein the outerlayer region is a first outer layer region, the multilayer fuel regionfurther comprising a second outer layer region, the second outer layercomprising a material selected to inhibit or moderate nuclear fissionreactions occurring in the nuclear fuel material.
 24. The method ofclaim 21, wherein forming at least one layer of the multilayer fuelregion comprises a chemical vapor deposition process.
 25. The method ofclaim 21, further comprising forming a plurality of discrete multilayerfuel regions along the at least one fiber of the plurality of fibers,wherein the multilayer fuel region is one multilayer fuel region of theplurality of discrete multilayer fuel regions, the plurality of discretemultilayer fuel regions comprising a respective inner layer regioncomprising the nuclear fuel material and a respective outer layer regionencasing the nuclear fuel material.
 26. The method of claim 21, whereinthe facilitating fabricating further comprises: providing an inner rodor tube structure of the nuclear fuel structure; arranging the pluralityof fibers to wrap around the inner rod structure so that the pluralityof fibers provide structural reinforcement to the nuclear fuelstructure.
 27. The method of claim 21, wherein the facilitatingfabricating further comprises: providing an outer tube structure of thenuclear fuel structure; arranging the plurality of fibers to wrap insidethe outer tube structure so that the plurality of fibers providestructural reinforcement to the nuclear fuel structure.